I'm all for an atomic drive of some discription. But thinking of the bomb drive, could we not use just very small ones. How small can you make a nuke? Keep tossing a mini nuke every half sec or so into the propulsion chamber? Sadly I have much of a clue about this, so will bow to others superior knowledge.

Erm, and yes, you'd only want to use it where nobody else would be affected.

There is a critical mass below which a runaway fission reaction cannot occur. The actual mass is, of course, officially secret, but an estimate of around 10 kg would probably be close to the mark. Even that mass would probably have a yield of more than a kiloton. So a really small nuclear explosion, in the usual sense, could not occur.

The heat sink version of Dyson's starship would have a pusher plate made of copper. This would require 5 million tonnes of exposed surface to absorb and then reradiate 1 megaton of bomb energy. The plate would have to be 20 km in diameter. 15 billion kg of deuterium would be used in 30 million bombs. The spacecraft would have an empty weight of 10 million tons and a loaded weight of 40 million tons. The bombs would explode 120 km behind the pusher plate at 1000-second intervals over 500-year acceleration and deceleration periods. Payload would be several million tonnes, enough to house a city of 20,000 people. The starship would reach a velocity of 1000 km/sec. Effective exhaust velocity would be 150,000 seconds. The cost would be equal to the entire Gross National Product of the United States. This generation ship would take 1800 years to reach Alpha Centauri. Total Mass: 40,000,000,000 kg. Core Diameter: 20,000.00 m

Uhh... wow. We should build a dyson sphere while we're at it.

Quote:

"The ablative version of Dyson's starship would be smaller and faster then the heat sink version. It would have a mass of 100,000 tons unloaded and be equipped with 300,000 one megaton bombs. It would accelerate in ten days to 10,000 km/sec, a velocity of 1 parsec per century. It therefore could reach Alpha Centauri in 130 years and would cost only one tenth of a GNP, or $ 150 per kilogramme of payload. Total Mass: 500,000,000 kg."

3.3% lightspeed... cool! Still too expensive though.

Quote:

"The final iteration of the Orion design was a nuclear pulse propulsion module launched into earth orbit by a Saturn V. The 100 tonne unit would have had a diameter of 10 m to match that of the booster. This would limit specific impulse to 1800 to 2500 seconds, still two to three times that of a nuclear thermal system. A second launch would put a 100 tonne Mars spacecraft with a crew of eight into orbit. After rendezvous and checkout, the combined 200 tonne spacecraft would set out on a round trip to the Mars - total mission duration as little as 125 days! Payload: 100,000 kg. to a: Mars and back trajectory. Total Mass: 100,000 kg. Core Diameter: 10.00 m. Total Length: 50.00 m."

Hey, we could actually build this one! Oh that's right, the Saturn V...

There is a critical mass below which a runaway fission reaction cannot occur. The actual mass is, of course, officially secret, but an estimate of around 10 kg would probably be close to the mark. Even that mass would probably have a yield of more than a kiloton. So a really small nuclear explosion, in the usual sense, could not occur.

Uh, no. No such thing. Take a slug of Uranium 235, fire it into a hunk of the same. This is how the first atom bombs were built. You don't actually need to reach critical mass -- that's when the thing goes off by itself. You want a controlled detonation -- you pick when it starts. So you'd have a mass driver firing Plutonium slugs (much more powerful than Uranium, but it has the nasty habit of reacting with air) at pieces of Plutonium that are dropped behind the ship in order to detonate the Plutonium and create an explosion to drive the ship forward. This is what is considered an Orion drive.

(EDIT: In doing the research for this post I found alot of contradictory information. After looking around more I fixed up my post)

Garnetstar is right about there needing to be a minimum mass to sustain a chain reaction, as that's what critical mass means, but he is wrong about it being secret. 50 kilograms is the amount of U235 that would be needed to achieve critical mass. Supercritical mass is when the amount of fissionable material is greater than the amount needed to sustain a fission chain reaction, the result is an exponential climb in the fission rate, resulting in a huge amount of energy being released and a very large explosion.

spacecowboy, you are refering to a gun-type detonator like the one used in Hiroshima where a cannon fires a subcritical mass of U235 into another subcritical mass of U235 creating a supercritical mass. This worked but it wasn't nearly as efficient as the next method and it only had an of efficiency 1.5%. It's interesting to note that because its design is so simple, this type of bomb was never tested before being dropped. Gun-type bombs only work with uranium, and not plutonium as the spontaneous fission would blow the fuel apart the moment the pieces touch, before any significant number of reactions.

The first bomb at Trinity and the Fat Man bomb that was dropped over Nagasaki used plutonium and instead of being gun-type it would use implosion. A sub-critical sphere of plutonium was surrounded by high explosive and when detonated it would crush the plutonium and become supercritical. How can something go from subcritical to supercritical by just being crushed, it can't gain mass can it? Well since the explosives compact the sphere, its density is increased and takes less plutonium to be a supercritical mass then it would at normal density. A Beryllium/Polonium core that was placed in the center of the plutonium sphere would emit neutrons, thus starting the chain reaction. This method achieved 17% efficiency.

The term "critical mass" is itself somewhat of an oversimplification, since density is also a factor. The explosion happens when enough neutrons are absorbed to cause fissions, while the material is contained long enough for a substantial fraction of the material to fission at once. Otherwise there can be fissions, but no explosion--that's what happens in reactors. Critical reactions can occur at low density--you may recall those Japanese workers that were killed when some sloppily-handled went critical. The density was low, but there was a lot of material. At a sufficiently high density the mass is much lower, but I stand by my original statement in modified form--that the actual combination of mass and density necessary to get an actual explosion (as opposed to a chain reaction) remains a closely-guarded secret (which, however, is the subject of some fairly accurate guesses). All in all, I still suspect that forcing a working fluid through a superheated reactor will be a better use of fission energy than the Orion approach.

Garnetstar, you would just have to know enough mathematics/physics and work out the numbers yourself. The people who worked on the Manhatten project figured out the numbers so it's nothing magical, or something that can be kept secret.

Very true, but officialdom still considers it secret. An interesting sidenote: attempting to determine how easy it would be for a competent physicist to work out all the information needed for the bomb, the military officials asked George Gamow to try--and were horrfied to see that he got everything right. Of course, some bomb development took place through overkill--going to a higher mass/density factor than was strictly necessary. But historically, the biggest problems were the practical ones--purifying the fissionable material to a high enough degree. compressing the material sufficiently, and containing the reacting mass long enough for a large percentage of the material to fission before it blew apart.

There is a critical mass below which a runaway fission reaction cannot occur. The actual mass is, of course, officially secret, but an estimate of around 10 kg would probably be close to the mark. Even that mass would probably have a yield of more than a kiloton. So a really small nuclear explosion, in the usual sense, could not occur.

Uh, no. No such thing. Take a slug of Uranium 235, fire it into a hunk of the same. This is how the first atom bombs were built. You don't actually need to reach critical mass -- that's when the thing goes off by itself. You want a controlled detonation -- you pick when it starts. So you'd have a mass driver firing Plutonium slugs (much more powerful than Uranium, but it has the nasty habit of reacting with air) at pieces of Plutonium that are dropped behind the ship in order to detonate the Plutonium and create an explosion to drive the ship forward. This is what is considered an Orion drive.

No. An Orion drive is rapid firing of individual COMPLETE nuclear bombs to a preprogrammed distance behind the pusher plate. The bombs are embedded in polyethylene plastic so that when each bomb detonates it vaporizes the plastic shell creating a mass of hot gas which pushes the vehicle (remember in space there is no atmosphere for the bomb to heat). Part of the still classified Orion technology was learning how to focus the explosion, allowing the diameter of the pusher plate to be radically reduced.

The other problem is the feasibility of constructing an appropriate exhaust nozzle. Either you have to explode the bombs a good distance away from the rocket (huge waste of power), or you need a combustion chamber that can contain and direct a nuclear explosion. The walls of the chamber would have to handle both heat and mechanical stress. Some ceramics have outrageous melting points (the CRC Handbook lists Tantalum Carbide as having a melting point of 4880 Celsius!), but they have a way of being brittle. The Columbia disaster shows that high temperature resiliance and high stress resiliance are not the same thing.

Project Orion used a pusher plate not an exhaust nozzle. The plate was to be covered with either graphite plates or a layer of graphite grease which would be extruded onto the surface.